Genomes and subgenomic clones of herpesviruses, human cytomegalovirus (HCMV) and human herpesvirus-6 (HHV-6), were studied for oncogenic potential in order to evaluate the role of these viruses in certain human cancers, especially the role of HHV-6 in lymphomas and lymphoproliferative disorders. CMV: We previously identified two transforming domains, mtrII (980 bp) and mtrIII (7.5kb) within the CMV genome by introducing viral DNA clones into rodent cells (NIH 3T3 or Rat- 2), and assaying the selected cells for tumorigenicity by growth in agarose or in nude mice. The transforming activity of mtrII was mapped to a 79-amino-acid open reading frame (ORF). Recently, we identified promoter activity by a choramphenicol acetyl transferase (CAT) assay and specific mRNA encoded by mtrII in 3T3 cells. In addition, we mapped the transforming domain, mtrIII, to a 270-amino-acid ORF. A 89-amino- acid ORF was required to maximize the transforming efficiency. These ORFs are currently being cloned into expression vectors to facilitate studies on the mechanism of transformation by these proteins. The CMV major immediate early gene promoter and the gene segment did not induce transformation. HHV-6: Similar studies were performed with HHV-6 to define its role in human lymphomas. Two DNA segments (21 kb and 8.7 kb) of GS strain ( isolated from a patient with lymphocytic leukemia) were found to transform NIH 3T3 cells and human keratinocytes. Other HHV-6 isolates from other patients could also transform 3T3 cells. Transformed 3T3 cells induced tumors with metastatic potential in immunocompetent mice. Cytotoxic T cells derived from induced tumors could lyse autologous tumors. In a separate study, HHV-6 DNA homologous to the 8.7 kb segment was identified in tissues from a patient with immunoblastic lymphoma in the absence of detectable EBV DNA. Recently, six additional Non-Hodkin's lymphomas examined were positive for HHV-6 DNA by PCR and two of them were positive for HHV-6 antigens by immunohistochemistry. HHV-6 may contain more than one transforming gene.These studies are important for the development and safety evaluation of CMV and other herpesvirus vaccines, and the development of herpesvirus-based vectors for gene therapy. Identification of a tumor antigen may lead to a specific diagnostic reagent or a vaccine for CMV- or HHV-6 induced malignant diseases.